Pebble bed reactor (PBR) is claimed to be have excellent passive safety features among the reactors. Rock-like oxide (ROX) fuel was proposed at JAEA with high chemical stability under geological condition. It is of interest that using ROX as fuel in PBRs can improve safety features during normal operation and the final disposal of spent fuel. The purpose of this study was to show the design concept for a small modular pebble bed reactor with ROX fuel that could achieve a high safety feature of spent fuel with high burnup performance. The results showed that the PBR with ROX fuel could become critical even though the ROX fuel’s density is low compare to conventional UO2 fuel. Also, by optimizing the fuel composition and core geometry, we could design a 120-MWth OTTO cycle small PBR that could achieve a high burnup of about 145 GWd/t-HM and a FIFA value of 75% while still satisfying with the safety requirement for HTGRs.